Polyaluminum chlorides (PACls) and polyaluminum chlorosulfates (PACSs) are used in water treatment and in the production of paper, antiperspirants, foods and pharmaceuticals. In wastewater treatment processes, they serve as flocculants and coagulants. Processes for producing polyaluminum chlorides and polyaluminum chlorosulfates with characteristics favorable for water treatment have been described in a number of previous publications, including: U.S. Pat. No. 5,246,686; U.S. Pat. No. 4,981,673; U.S. Pat. No. 5,076,940; U.S. Pat. No. 3,929,666; U.S. Pat. No. 5,348,721; U.S. Pat. No. 6,548,037; U.S. Pat. No. 5,603,912; and U.S. Pat. No. 5,985,234.
Low basicity PACls (0-45% basicity) are very stable in solution but can also be very corrosive. They tend to form small flocs, depress pH and require a high concentration relative to other products used in water treatment. High basicity PACls (45-75% basicity) are somewhat better as coagulants in water treatment, but can cost more to manufacture and have a limited shelf life. Highly basic aluminum chlorohydrate (83% basicity) solves most issues of its lower basicity counterparts, but is not effective as a coagulant in cold water or highly turbid water.
In contrast, PACSs are highly efficient coagulants and work well in either cold or highly turbid waters. The main problem with these compounds is that they have a limited shelf life and degrade rapidly at elevated temperatures. PACSs may be made by shearing sodium aluminate into a basic aluminum chlorosulfate solution. However, this produces as much as 5-10% by-product sodium chloride and sodium sulfate which becomes an impurity in water treatment operations and results in an elevation of the chloride and sodium content of the treated waters. The by-product salts also reduce the concentration of the PACSs and shorten the shelf life of these products.
As an alternative, PACS may be made by mixing lime with a mixture of aluminum chloride and aluminum sulfate solutions. In this case, by-product calcium sulfate or gypsum are produced which need to be disposed of. It is also difficult to produce basicities of greater than 50% using this technology.
As mentioned previously, solutions of PACls or PACSs are often used in water treatment procedures. However, waters with a high chloride-to-sulfate mass ratio (CSMR) can cause galvanic corrosion of solder and thereby create higher lead levels in drinking water (Edwards, et al., JAWWA 99(7):96-109 (July 2007)). Use of all chloride PACls and low sulfate PACS can make this problem worse, as can PACSs that contain by-product sodium chloride.